It’s snail and slug season. The slimy mollusks are most active when temperatures are mild and rain or irrigation provide ample moisture. They can be bad pests in a garden, but some are good and a few can be dangerous.

The good

The predatory decollate snail feeds on eggs and young of the brown garden snail and other species. It has been used effectively in citrus groves throughout Southern California and in freeway plantings of ivy and ice plant ground cover. Decollate snails feed mainly on decaying vegetation, but home gardeners should release them with caution since they can damage young seedlings and new herbaceous transplants.

The bad

The most common snail in gardens and landscapes is the brown garden snail (Cornu apsersum, formerly Helix aspersa). Two other species, the white garden snail and milk snail, are also established in parts of San Diego County.

These terrestrial snails hide during the day and feed mainly at night on a variety of herbaceous and woody plants. They chew holes in leaves, flowers, fruit and can also damage thin bark on trees. Snails and slugs travel on a muscular “foot” that excretes slimy mucus that dries to a silvery trail.

A few methods have traditionally been used to control snails and slugs:

• Hand picking can be very effective if it is done often (daily at first, then weekly). If the soil is dry, water plants in late afternoon to draw out hiding mollusks. Return after dark with a flashlight and crush or collect any snails or slugs you find. Put them in a sealed plastic bag and discard in the trash.

• Traps make collecting mollusks easier. Snails will hide under a wide board raised off the ground with 1-inch high runners and slugs will hide under boards placed directly on the ground. Place several traps in planted areas and check them during the day.

• Copper barriers repel snails and slugs. Snails will not readily cross a wide band of thin copper flashing or foil (Snail-Barr) attached around the trunk of a tree or on the wooden frame of a raised bed.

• Toxic baits are effective when snails and slugs are active. Water first if soil is dry, then scatter bait lightly on soil around plants. Bait with iron phosphate is safer for animals and children than bait with metaldehyde.

The ugly (and dangerous)

Rats, snails and slugs are primary hosts of the rat lungworm, a parasitic nematode Angiostrongylus cantenosis. In humans it causes a serious, painful and sometimes fatal disease, Eosinophilic meningitis. People can acquire the disease by eating raw vegetables such as lettuce, fruit or water contaminated by nematode carrying slugs, snails or their slime or by eating raw or undercooked mollusks and prawns.

Rat lungworm is present in Hawaii and other tropical areas of Asia, the Caribbean and Central/South America. It was also found recently in Florida in an aquatic snail. Produce grown in infested areas should be thoroughly washed to remove any mollusk slime, snails and slugs (as small as 1/10 of an inch long) which may contain the nematode. The parasite does not appear to penetrate unbroken skin, but it is still a good precaution not to handle live snails or slugs and closely supervise young children while visiting areas infested with rat lungworm.

Vincent Lazaneo is an urban horticulture adviser emeritus with the University of California Cooperative Extension. Send questions for “Plants & Pests” to homeandgarden@utsandiego.com